\)

Velocity distribution of ion beams from the
RIKEN Ion-Guide Isotope Separator On-Line
Mitsuo Koizumi

Department of Physics, Faculty of Science
Hiroshima University, Higashi-Hiroshima 724, JapanAbstract
The nuclei close to the N = 104 show an interesting feature. Extremely large
shape staggering in the isotope and isomer shifts was observed for Hg and TI isotopes ,and shape coexistence at low excitation energy was observed for Hg, Au, and Pt isotopes.
It should also be noted that many high-spin isomers exist in the nuclei of this region,such as Hf, Wand Os. For many elements in this region, however, laser spectroscopy
has not been reported, because these elements are refractory ones. At R1KEN, we haveconstructed an 1SOL system GAR1S/1GISOL which is powerful to extract radioactive
isotopes of refractory elements. Taking full advantages of this feature, we aim at performing collinear laser spectroscopy to study exotic nuclei of these elements. To make on-line
collinear laser spectroscopy, the radioactive beams from the GARIS/IGISOL is required
to have adequate quality (velocity spread) and quantity. Therefore, direct measurement of
the velocity spread of Ar 1+ ion beams from the IGISOL was carried out with a techniqueof laser spectroscopy. The skimmer-potential and the gas-cell pressure dependences ofthe velocity spread were systematically investigated. A new focusing device SQUEEZER
was introduced to reduce the velocity spread, and the effect of this was experimentally
examined. From these measurements, it was pointed"out that the ion beam from the
GARIS/IGISOL has adequate quality for making collinear laser spectroscopy. An estimation of the yield of radioactive isotopes from the GARIS/IGISOL was also made fora fusion reaction 9Be(166 Er, xn)175-X H f. The estimated yield was expected to be morethan 10 4 particles/so The yield of the radioactive isotope is adequate to make collinear
laser spectroscopy with a coincidence method. We have for the first time in the world
extracted the radioactive isotope of refractory element 169Hf with the GAR1S/1G1SOL.
We conclude, thus, that the collinear laser spectroscopy of refractory elements with the
GARIS/IGISOL is promising.

Contents
1 Introduction

1

2 Principle of measurement

5

2.1

Doppler shift and Doppler broadening .

5

2.2

Mass resolving power (MRP)

811

3 Experimental
3.1

Location of IGISOL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.2 SQ UEEZER . . . . .

4. . . . ..

11
11

3.3

Discharge ion source

14

3.4

Laser system

18

3.5

Experimental setup for velocity measurement

18

3.6

Production of radioactive isotope of a refractory element: 169 Hf .

22

....

Results and discussion

24

4.1

LIF spectrum of Ar 1 + .

.. . . . . . . . .

24

4.2

Velocity spread and beam divergence. . . . . . . . ..

24

4.3

Skimmer-voltage dependence of the velocity spread. . . . . . . . . . . ..

27

4.4

The effect of SQUEEZER. . . . . . . . . . . . . . . . . . . . . . . . . . .

28

4.5

He-gas pressure dependence of the velocity spread . . . . . . . . . . . . ..

29

4.6

Comparison with the mass resolving power . . . . . . . . . . . . . . . . . .

30

4.7

Velocity distribution of a collimated beam.. . . . .

35

4.8

Mass dependence of the MRP . . . . . . .

. . . . . . . .

35

Yield of radioactive isotopes from the GARIS/IGISOL

37

4.10 Feasibility of on-line collinear laser spectroscopy . . . .

38

4.9

5 Conclusion

41

Appendix

43

A

Collinear laser spectroscopy setup for on-line experiment43

B

High-resolution off-line laser spectroscopy setup for refractory elements

48

Acknowledgment

52

References

53

II

1

Introduction
The existence of atomic nuclei is reflected in the atomic energy levels througha small but well-defined electromagnetic interaction [I, 2].

The precise study of theatomic spectra, therefore, provides fundamental information on nuclear ground-state andisomeric-state properties without using any nuclear model: changes of mean-square nuclear charge radii (0 < r2 » are deduced from isotope shifts; nuclear spins (I) and nuclear electromagnetic moments (p"Q, ... ) are deduced from hyperfine structures. IIigh-resolution laser spectroscopy has successfully been made to study these nuclear properties [3, 4, 5].
The accessible area on the chart of the nuclides is extending further and further
away from the fi-stability line with development of an accelerator and an isotope separator on-line (ISOL). To study the basic properties of exotic nuclei, the method of laser
spectroscopy was combined with the ISOL technique [6, 7]. For many elements, valuable know ledges of the nuclear structure have been obtained systematically for many longisotopic chains [6, 7, 8].
For the mercury nuclei close to N = 104, extremely large odd-even staggering in
the isotope and isomer shifts has been observed [8]. This staggering is understood as achange of the nuclear shape, i.e., oblate-prolate shape staggering [8]. This staggering wasquite an unexpected feature for nuclei very near to the spherical closed-shell Z = 82. Following this discovery, the coexistence of different shapes, well-deformed and near-spherical,
was observed for mercury isotopes by means of systematic in-beam, spectroscopy [9]. It
is also surprising that the nuclei have second minima of their potential-energy surfaces at
low spin and low excitation energy, and the potential barrier between the first (ground1

state) and second minima is high enough to build complete bands. For Au and Pt isotopes, the shape coexistence was also observed [10J. In this region, the potential-energy
surface of nuclei changes drastically as a function of neutron and proton numbers. Therefore, it is interesting to study the nuclear shapes systematically in such a transitional
region. Moreover, it should be pointed out that high-spin isomers exist in the transitional
region, for instance, in nuclei of Hf, W, and Os. It is also interesting to know the shapesof high-spin isomers, because there are two possibilities to generate such high-spins: One
is the alignment of a few nucleons along the symmetry axis and the other is collective
motion. The former is called high-K isomer and the latter shape isomer.
Laser spectroscopy is suitable to study nuclear shapes of ground and isomericstates systematically. For TI, Au and Pt elements, isotope- and isomer-shift measurements have already started [11, 12, 13]. For refractory elements such as Hf, W, and Os,
however, on-line collinear laser spectroscopy has never been reported, because it is hard to
extract these elements from a conventional isotope separator equipped with a heated-ion
source [14].
An ISO L system, which consists of a gas-filled recoil isotope separator (GARIS)
and an ion-guide isotope separator on-line (IGISOL), has been constructed being combined in tandem at RIKEN Accelerator Research Facility [15, 16]. This system has noheated-ion source, so that it provides us great possibility to study the nuclei of refractoryelements on-line.
The IGISOL was first developed at the University of Jyvaskyla [17]. vVith thissystem, they actually showed that short-lived isotopes of the order of one millisecond
half-life could be extracted even though they are refractory elements [18]. Therefore, the2system GAR1S/1G1SOL is considered an extremely powerful tool in separating refractoryelements. In fact, a radioactive isotope of refractory element, 169Hf, has been extracted
from this system [19]. We now aim at performi ng collinear laser spectroscopy to study
radioactive isotopes of refractory elements with the GARI S/IGIS O L. On the other hand, the mass resolving power (MRP) of an 1G1S O L was reported
to be lower than that of the conventional 1SOL. The energy spread of t he IGIS O L ion
beam deduced from the MRP was more than 100 e V [20, 21, 22]. For the conventional
ISOL, the energy spread is less than 10 eV [14] , where collinear laser sp ectroscopy has
been established. This suggests that the ion beam from the IGISOL is inadequate to docollinear laser spectroscopy. It should be pointed out, however , that the estimation of
the energy spread of the IGISOL ion beam was made on the assumption that t he MRP
depends only on the momentum spread of the ion beam. To discuss the feasibili ty of laser
spectroscopy using the GARIS/IGIOSL, it is important to measure directly the velocity
spread of ion beams from the IGISOL.
Laser spectroscopy is ideally suited for measuring the velocity and its distributionof an atomic beam, because the resonance spectrum reflects the beam quality along th e
direction of the laser beam as the Doppler shift and broadening. From a measure me nt at
collinear geometry, the longitudinal velocity and its distribution are deduced, and fro m a0measurement at 90 incidence, the transversal velocity distribution or beam divergence isdeduced. In principle, this method is applicable to various elements in the form of atoms ,ions and molecules.
At a relatively low acceleration potential, direct measurement of the veloci ty
distribution of an Arl+ ion beam by means of collinear laser spectroscopy was performed3

by N. Schmidt at Mainz university [23]. In the light of his experiment, we have measured
the velocity distribution of the Ar 1 + ion beam from the IGISOL for the first time. It
was pointed out that the MRP depends on the skimmer potential and pressure in the gas
cell [20, 21, 22]. Therefore, we measured the velocity distribution of the ion beam fromthe IGISOL systematically, changing the condition of the IGISOL.
It is also important to produce an adequate number of radioactive isotopes forcollinear laser spectroscopy, because a beam intensity of more than 10 4 particles/s is required [7]. The efficiency of the GARIS/IGISOL has been experimentally determined [16].
According to the previous results [16], the estimation of the yield of radioactive isotope
from the GARIS/IGISOL has been made, and the production of the unstable nuclei 169Hf
has been experimentally done.
In this paper, we shall present the results of our velocity spread measurement,
and discuss how to obtain the ion beam with a narrow energy spread. We shall also
show the estimated and experimentally obtained yield of radioactive isotopes from the
GARIS/IGISOL. Finally, the feasibility of laser spectroscopy with the GARIS/IGISOL
will be discussed in detail.

4

2

Principle of measurement

2.1

Doppler shift and Doppler broadening
Laser spectroscopy is a powerful method to measure the velocity distribution of

atomic beams. The Doppler shift and broadening are the reflection of the velocity of theatomic beam and its spread along the direction of the laser beam. The relation between
the resonance frequencies of the atom in the rest frame (vo) and in the laboratory frame(v) is given byv

VoVI - 132
1 - 13 cos ()Vo ( 1 + 13 cos () -"2113 2 + 13 2 cos 2 ()... ) ,(1 )where () is the incident angle between laser and ion beams, and 13 = v / c. (See fig. 1.)
The velocity of the atom is given by v and c is the velocity of light. The Doppler shift v D
is then written asVD

1 2 + (3 2 cos 2 ()... ) .
= Vo ( 13 cos () -"2(3(2)

The Doppler broadening 6VD is caused by the velocity distribution ±8(3 and the angulardispersion ±8(). Here we define the sign 6 as the full width at half maximum (FWHM)
of any distribution and the sign 8 as the half width at half maximum (HWHM). The
quantities 813 and 8() can be treated independently, so that we have5Figure 1: The relation between the laser and ion beams.

(3)

where ~v/3 is a Doppler spread due to the velocity distribution and ~v(J is the one due tothe dispersion of the ion and laser beams.
To first order of (3, the Doppler shift is given byVn= vo((3 cos e).

(4)

The contributions to the broadening is then written as~Vf3

2voo (3 cos e,
2vo(3oe sin e.

6(5)For a collinear geometory (180° incidence), ~ vo = 0, and we obtain the Doppler broadening~v = 2voo (3 .

(6)

For 90° incidence, ~v{3 = 0, and we obtain~v =2vo(308.

(7)

As shown in eqs. (6) and (7) , the velocity distribution can be deduced from a collinear
measurement, and the angular distribution from a measurement at 90° incidence.
To second order of (3, the Doppler shift is given by(8)

Contributions to the Doppler broadening are given by, to second order of 0(3 and 08,

2vo ((308 sin 8 + 2(3208 cos 8 sin 8+~,8oe2 cos e + ,8261]2 (1 - 2sin2 e) ) .

7(9)

The velocity is so small t hat the kinetic energy E is safely given by the nonrelativistic expression

1

2

2

E = eU = - me /3 .
2

( 10)

The energy spread tlE is therefore(11 )and we have

flE
flU
fl /3
- = - = 2 -.
E
U
/3

(12)

For collinear m easurement we have, to first order of /3,

flE
flU
flu
- = - = 2 -.

E

2.2

U

UD

(13)

Mass resolving power (MRP)
The relation of the magnetic field B of an analyzing magnet and an an alyzed ion

beam with mass M and a kinetic energy E was given by8

qBp = P = v'2M E,(14)where p is a radius of the central ray of the analyzing magnet and q charge of the ion. The
6.B of mass spectrum corresponding to the 6.M as shown in fig. 2 is, therefore, written

as

(15)

The value 6.M is a reduced value from the energy spread 6.E. Therefore the relationshipbetween the MRP and the energy broadening is

MRP- ~- 6.M -

(£)
- (~)
6.E
6.U
M -

M·

( 16)

Here, we should bear in mind that this relation is valid only for ideal beam optics, i.e.,
no emit tance is taken into account.

9

M

M-l

Magnetic field [B]Figure 2: A mass spectrum observed as a function of magnetic field B of the analyzing
magnet. The value of !:::.M is a FWHM of the mass spectrum associated with !:::.B.

10

3

Experimental

3.1

Location of IGISOLThe RIKEN ISOL system, which consists of the GARIS and the IGISOL, is shownin fig. 3 [15, 16] . The GARIS has three magnets arranged in a D-Q-Q configuration, andthe IGISOL is connected in tandem with the GARIS. The components of the IGISOL
are an ion-guide chamber, an acceleration section, a beam transport section, a magnetic
analyzer, an electrostatic mass analyzer and three beam lines. The 45° and 90° beamlines are designed for point-to-parallel and point-to-point beam optics, respectively. The
cross-section of the ion-guide chamber is shown in fig. 4.
The purpose of the GARIS is to separate recoil products from the incident beamand to prevent the beam from entering the gas cell. This is to avoid a plasma effectthat causes neutralization of radioactive ions in the gas cell [24] . At the focal point of
the GARIS, the transported radioactive isotopes are stopped in the gas cell of the ionguide chamber with an energy degrader and a helium buffer gas. The stopped radioactive
products are cooled down and thermalized by collisions with helium atoms. A part of the
products survives as singly charged ions in the gas cell. They are guided by a helium gas
flow to the skimmer and thus extracted by electric fields of the skimmer and the extractor.

3.2

SQUEEZER
It is known that higher MRP is obtained for the lower skimmer potential [22].

However, the yield of the radioactive isotope extracted from the IGISO L decreases for the
lower skimmer potential [22]. To obtain high MRP without losing the efficiency of IGISOL,a group of Jyvaskyla developed a new focusing device SQUEEZER [25]. The SQUEEZER

11

Target Pos ition (Mode II )(Scattering Chamber )o2Target Pos ition (Mode I )3m

I =====c==========~!
~GAR ISIGI SOLlon- uide Cha mb er
Beam Profi le Monitor

Extraction Chambe r
Diffusion Pump

Einzel Lens
Electrostat ic Q- Ien s

Mechanical Booster
Pump(doublet)

IG ISOLD

Figure 3: A layout of the R1KEN 1SOL, GAR1S/1G1SOL .

12

Ski m me r
Extractor

5000 m3 / hr
MoBoPoThermalization Chamber( as cell )Einzel Lens

Ion Beams... from GARIS

145

J;•

70

Figure 4: An ion-guide chamber housing a gas cell with an energy degrader, a skimmer,
an extractor and an einzellens.

13

is set between the gas cell and skimmer as shown in fig. 5(a). The SQUEEZER has three
grids. To the first grid, a low positive potential is supplied. The positive ions coming fromthe exit hole of the gas cell are pushed by the helium gas flow and climb the potential hillproduced by the first grid. In this process, the ion beam slows down and is focused to thecenter. The second grid is used as a pre-accelerator and the third grid as an einzel lens.
An electrostatic potential given by grids of the SQUEEZER is shown in fig. 5(b). This
device makes it possible to extract ions with a low skimmer potential such as V.sk = 10
V and a high transmission efficiency of 70 % [26]. The energy spread of an extracted ion
beam was measured and found to be 2.3-3.5 e V (FWHM) at Jyvaskyla university [27].

3.3Discharge ion source
To study the performance of the IGISOL without incident beams (off-line), a large

amount of ions should be provided in the gas cell. The discharge method, which has been
carried out at Tohoku university, is convenient for producing ions of atoms and molecules
mixed with a helium buffer gas in the gas cell [21]. Especially, a gaseous element is easily
mixed with a helium gas, while an additional device is required for a metal element, whichacts as an oven and evaporates the metal in the gas cell.
Collinear laser spectroscopy was carried out for the metastable state of Li 1 +and Arl+ by N. Schmidt at Mainz university [23]. For our velocity measurement, laser
spectroscopy of ions (not neutral atoms) is suitable because there is no need to neutralize
ion beam using a charge exchange cell.

And it is much easier to handle gaseous Ar

than to handle active Li as mentioned above. Therefore in the light of the Schmidt's
experiment [23], we have measured the velocity spread of Arl+ ion beam from the IGISOL.
The schematic diagram of the ion-guide chamber housing discharge electrodes is14

(a) SQUEEZER

Skirnnler
SQ UEEZ ER

Exit Hole

7.
~

III
III

Ion beams
from GARIS

Gas CellV sk(b) Electrostatic potential producedby the SQUEEZER

VI

-----

15

u

V2 -------------Vsk-------------------------------

Distance from the gas cell [mm]Figure 5: (a) Schematic diagram of SQUEEZER set in the ion-guide chamber. (b) Electrostatic potential associated with the SQUEEZER. The potential of the gas cell is taken
as 0 V. The abscissa is a distance from the exit hole of the gas cell. The positions of gridsof the SQUEEZER and the skimmer are given by Xl, X 2 , X 3 , and X$}q respectively.

15

Skimmer
Acceleration
Electrodes
Thermalization
Chamber (Gas Cell)

Ion Beams
Flow
Controller
Slow Leak
Valve

00
Ar gas

He gasFigure 6: A schematic diagram of the ion-guide chamber with a discharge ionization
device.
shown in fig. 6. A small amount of the argon gas was mixed with the helium gas throughthe slow leak valve (ANELVA 951-7172). Inside the gas cell, Ar atoms are ionized by aglow
discharge between the two electrodes. The singly charged Ar ions Arl+ are extracted andaccelerated under usual IGISOL operation. The size of the exit hole of the gas cell was 1.1
mm in diameter and the skimmer was 1.5 mm in diameter. The produced ions increased
logarithmically as a function of the discharge current. Figure 7 shows an example of theperformance of the discharge ion source.

16

10 3
10 2
,---,

~

c:

10 1

\..........J

~

.~

V'l

10 0

c:

<l)
~

c:

~

10 -1
10 -2
2.0

4.0

8.0

6.0

10.0

12.0

Discharge current [InA]

Figure 7: Discharge current dependence of the ion beam current. The beam current
extracted from the IGISOL increases logarithmically as a function of discharge current.
The pressure in the gas cell was 20 mbar, and the potentials at electrodes were ~k == -500
V, Vex == 10 kV, Vel == 5.7 kV, and Vacc == 30 kV.

17

3.4

Laser system
A laser beam used was produced by a cw ring dye laser (Coherent 699-29), which

was pumped with an Ar ion laser (Spectra Physics 171-19). The dye used was Rhodamine6G, which is applicable to produce a laser beam varying in a wavelength range of 550 nm
f'.J

650 nm. The laser beam was transported through a 160-m optical fiber (Mitsubishi

ST50A-SY) from the laser room to the experimental vault.

The overall transmission

efficiency of the optical fiber is about 70 %. A part of the laser beam was sent to an 12cell and a Fabry-Perot interferometer to refer to the laser frequency. The pulse signal
from the photomultiplier are counted and recorded with a microcomputer (NEC PC9801). The spectra taken at the 12 cell and the Fabry-Perot interferometer were recorded
simultaneously with the laser induced fluorescence (L1F) spectrum.

3.5

Experimental setup for velocity measurement
The Arl+ ion, which was produced in the gas cell by means of the glow discharge,

was extracted, accelerated to 30 ke V, and transported to the laser spectroscopy setup
as schematically shown in fig. 8. At the focal point of the analyzing magnet, we have amass defining slit and a Faraday-cup, which are used for mass-spectrum measurement.
The mass-separated ion beam was deflected by an electrostatic mass analyzer. The L1F
observation section was set on the 45° beam line, which is designed for point-to-parallel
beam optics.

An electrostatic quadrupole doublet and an einzel lens were used as afocusing element. At the L1F observation section, the ion beam intersected the laser
beam at the angle of 30°, 90°, and 180°. The size of the ion beam at the interaction
region was about 0.5 cm in diameter, and the size of the laser beam about 1 cm in18

IGISOL
D-magne t

SlitsElectrostatic
Mass
Analyzcr

Faraday Cup
Photomultiplier

Acceleration
Electrodes

Spherical
Mirro rLaser BeamIon Beams from theion-guide chamber

Figure 8: A schematic diagram of the IGISO L beam transport system and the LIF observation section.
diameter. The laser power at the interaction region was typically 100 m W.
The 611.66-nm transition from the 3d' 2G9 / 2 metastable state to the 4p' 2 F7/2
state in Ar 1 + was observed. The resonance wavelength of the laser beam for collinear
geometry (180° incidence), for example, is 612.44 nm at U = 30 kV and the Doppler shiftof the resonance frequency is 622 GHz. The atomic energy levels of Ar 1+ associated withthe measurement are shown in fig. 9. Fluorescence photons emitted from the 4p' 2 F7 / 2
level was collected with a spherical mirror, which was 80 mm in diameter and 60 mm inradius of curvature. The solid angle of photon collection is 0.9S7f sr. The backgroundphotons are mainly stray lights from the laser beam and scattered lights from ions , which
is produced by the collision with residual gas during the flight in the vacuum chamber.
Because of the large difference between the wavelength of the laser beam and fluoresc ence19

4p' 2F....-.....----......- 7/2

611.66nm

514 .18nm

~7/2
~9/2
3d' 2G~o.96nm

--iF!
3d 1)

5/2
--'---7/2-~-5/2---3/24s' 2D

Figure 9: Atomic energy levels of Arl + associated with the measurement .
lights, a band pass filter effectively reduces the background photons. The 460.96-nm
transition from the excited state 4p' 2 F7/2 to the state 48' 2 D S/ 2 was selectively observed
because this transition is strongest [23]. (See table 1.) The band pass filters used were
Shott BG25 and Kenko BP-46, which were set just in front of the single-photon counting
photomultiplier (Hamamatsu R329-02).
For collinear geometry, the population of the ions in the metastable state rapidly
decreases during the flight in the resonance laser field because of optical pumping: theions in the metastable state are repeatedly excited by the resonance laser field and decay
to other levels. Therefore an additional acceleration electrode was set at the observing
section as shown in fig 10. This device prevents the optical pumping by changing the ion
velocity, i.e., the resonance frequency of moving atoms. Here we applied negative voltageof 100 V to the electrode.

20

Table 1: Transition probability from the excited state 4p' 2 F7 / 2 [23].

Configurations

Aki [10

3d' 2G9 / 2
48' 2 D5 / 2
3d' 2 D5 / 2
3d' 2 F5 / 2
3d' 2 F7 / 2

6

s-l]

24
74.4
26.9
8.07
2.2

fron1

Laser room

160-m
Optical FiberPM

Filtersi Spherical Mirror
Figure 10: A schematic diagram of the LIF detection system for a collinear geometry.

21

3.6

Production of radioactive isotope of a refractory element:

169Hf
Hf isotopes, which are of a typical refractory element, were chosen for the firston-line collinear laser spectroscopy because of the existence of the famous high-spin isomer.
In a previous paper [16], it was pointed out that the transmission efficiency of
GARIS depends strongly on the angular distribution of the recoil products.

Table 2shows efficiencies of the GARIS for different types of nuclear reactions. The differenceof the efficiency is mainly caused by the angular distribution of the recoil products. For
recoil products having lower momentum, however, the efficiency of GARIS is further
reduced by an energy loss and straggling caused by the multiple scattering with gas in
the GARIS. Accordingly, the inverse kinematics of a fusion reaction with heavy incident
beam is useful to produce radioactive isotopes because the recoil products have narrower
angular distribution, larger momentum, and narrower momentum spread than for thecase of normal fusion reactions.
The inverse fusion reaction 9Be(166 Er, xn)175-x H f was used to produce Hf radioactive isotopes. An incident energy of the 166Er beam was 16 3 Me V /nucleon and thebeam current was a few pnA. A 9Be target of 50-j..lm thickness was placed at the target
position II (MODE II, see Fig. 3). The GARIS was filled with a nitrogen gas of 10 mbar.
The produced Hf isotopes were transported to and focused at the ion-guide chamber thatwas filled with a helium gas of 600 mbar. Singly-charged ions extracted from the ion-guidechamber were accelerated to 30 keY. The accelerated ion beams of the Hf isotopes were
stopped with an aluminum foil and accumulated. After about 10-minutes accumulation, ,rays from the decay of Hf isotopes were measured with a Ge(Li) detector. The 492.9-keV

22

Table 2: Efficiencies of the GARIS for various reactions.

Reaction

169Tm(40 Ar, xn)
181Ta(40 Ar, axn)
na t W(14 N, X)169 H f

fusion reaction
fusion reaction
target fragmentation

Incident Energy

Efficiency

5 MeV/nucleon
6 MeV/nucleon
35 MeV/nucleon

15 ± 1 %5.3 ± 0.4 %0.5 ± 0.1 %! rays from the decay of 169Hf (Tl/2 = 3.26 m) were clearly identified. This is the firstmeasurement of isotopically separated radioactive atomic beams of refractory elements.

23

4

Results and discussion

4.1

LIF spectrum of Arl+0

Figure 11 shows a typical LIF spectrum measured for 180 incidence at a heliumgas pressure (Ptc ) of 100 mbar, a skimmer voltage (Vjk ) of -500 V and an acceleration
potential (U) of 30 kV. The scanning speed of the laser was 9 s/GHz and the dwell time
for photon counting 0.5 s/ch. The FWHM of the spectrum is about 1.4 GHz. We repeatedthe measurement ten times, and determined the FWHM to be 1.45 ± 0.06 GHz.

4.2

Velocity spread and beam divergence
The skimmer potential dependence of the linewidth of the LIF spectrum wassystematically measured for three incident angles (30

0

0

,

90 and 1800)

at a helium gas

pressure of 20 mbar. The results of the investigation are shown in fig. 12. The solid lines
are the linear least-squares fit for each incident angle as a function of V"k.
To first order of /3, it is enough to measure the linewidth of the LIF spectrum
at collinear geometry only to deduce the velocity spread of ion beams. We carefully
examined, however, the second order effects of the j3, 0j3 and 08. Since linewidths were
measured for three incident angles at a given V:k, the oj3 and the 08 are deduced using
least-squares fits to eqs. (3) and (9), which are the approximation to second order of 8/3
and 8e. The results of the fittings are plotted in fig. 13 . The open circles indicate 8/3//3
and open squares 8e.
For the 40 Ar 1 + beam with a kinetic energy of 30 ke V, the velocity of the ions is j3 =1.3 X 10- 3 and the orders of magnitudes of 8j3 and 8e are 10- 6 and 10- 3 rad. Accordingly, it
is concluded that the approximation to first order is sufficient to calculate the velocity and

24

2000

b

12 cell
>.

~

~

1500

crJ

cC1)
~

c

~

o

a
o~0
0
0

1000

0
0
0
0
0

0
0
0

0

e

0

0
0

0

0

0

0
0
0

0

~

500

)

0
0

0

I

I

50

I

I

100

150

I

200

250

2000----------------------------------~

LIF spectrun1

Ar 1+

o

cP
I)

>.

1500

o

0
(l

o

<0

FWHtvl=1.4 GHz

o

~

o

crJ

c:

C)

o

c:

o

......
~

o
o
00

1000

%

e

@

Channel(Laser Frequency [56 MHz/ch])Figure 11: LIF spectrum for the metastable state of Ar 1 + ions at 180 0 incidence. The
condition of the IG1S0L was as follows: ~k = -500 V, Ptc = 100 mbar and U = 30 kV.
A reference spectrum of 12 molecule was measured simultaneously. The wave numbers of
the peaks of 12 spectrum are 16328.1875 cm- 1 and 16328.3072 cm-I, respectively [28].

255.0 . - - - - - - - - - - - - - - - Ptc = 20 mbar~

1.0

o. 0 ~..I.....I.."""__J.......I_.I~.o.......L.....I._...I..."""__J.......I_..i~------...L._..._I._.J.__'__L......_.~........J..........

100

200

300

400

500

600

700

Skimmer Voltage [-V]

Figure 12: Skimmer potential (V,k) dependence of the linewidth of an LIF spectrumat three incident angles at Ptc == 20 mbar and U == 30 k V. The solid lines are linear
least-squares fits to the data as a function of V,k·
(X 10- 3 )4.0
o • 8~/~9o • 88
3.0

•

f-

0-.1

as

2.0

d. 01

f-

"'0

c:

~

- 1.0 -

c:J...
c:J...

•

-.L

c.o

0.0

I

100

200

300

400

500

600

700

Skimmer Voltage [-V]oeFigure 13: Skimmer potential dependence of 8f3 / f3 andat Ptc == 20 mbar. The open
circles and squares were deduced from measurements at three incident angles using leastsquares fits to second order. The closed circles and squares are derived directly from themeasurements at the incidence of 90 and 180 using the approximation to first order.
0

0

26

angular distributions, i.e., the effect of the angular distribution is negligible for collinear
measurements and the effect of the velocity distribution at 90° measurements.
Using the approximation to first order of {3, we obtained 5{3 and 58 directly fromthe 180° and 90° measurements, respectively. The closed circles and squares shown in fig.
13 are deduced in this way. There is no significant deviation between the plots deduced
from the approximation to first order and second order. Therefore we can safely derive5{3 from the data at the collinear geometry only.
In this measurement we found that 58 linearly increases with Y:k while 5{3 israther constant. The change of the beam divergence is due to the change of the emittanceof the ion beam caused by a lens effect of the skimmer. In other words, more scat teredions are collected at a higher skimmer potential.

4.3

Skilnmer-voltage dependence of the velocity spreadTo investigate the skimmer potential dependence of velocity spread at a usualpressure in the gas cell (100 mbar), we measured LIF spectra at collinear geometry. The
observed skimmer potential dependence is shown in fig. 14. Measurement at 20 mbar was
also carried out for comparison. From the measurements at Pte = 100 mbar, we foundthat the linewidth of the LIF spectrum decreases with decreasing the skimmer voltage
less than 500 V and saturation was observed at the skimmer potential more than 600 V.
For Pte = 20 mbar, the linewidth decreases linearly with decreasing the skimmer
voltage and saturation was not observed. Here we noticed a systematic shift in the measurements at Pte = 20 mbar between this measurement and that mentioned in section 4.3.
The difference between them was only the alignment of the IGISOL: the alignment of theexit hole, the skimmer and the extractor. This shows that the geometrical setup of the

27

2.0
0

0
r--1

N

e = 180

20 mbar
100mbar

0

150

1.5

:=

0

,--,

~

~

:=

100~

1.0

~

<J

~

~

50

0.5

200

400

600

800

o

1000

Skinlmer Voltage [-V]

Figure 14: Skimmer potential dependence of the linewidth of an LIF spectrum observedin the collinear geometry. A deduced instability of the acceleration potential (tlU) is
given on the right ordinate.
IGISOL sensitively affects to the velocity distribution. The scales of the presentations offigs. 12 and 14 are so different that the shift seems quite large. But the shift is actually
not so large, and does not affect the present systematic analysis.
As shown in fig. 14, an ion beam with narrower velocity spread was obtained fora lower skimmer potential. The efficiency of the IGISOL to extract radioactive isotopes,
however, decreases with reducing the skimmer potential [22]. Accordingly, it is not easy
to obtain intense radioactive ions with a narrow velocity distribution.

4.4

The effect of SQUEEZER
A new focusing device SQUEEZER enables us to extract ion beams with a low

skimmer potential without losing the efficiency of the IGISOL [26]. We have introduced
this device into our IGISOL and measured an LIF spectrum of the Arl+ beam from the28

IGISOL. The pressure in the gas cell was 100 mbar, and the voltages of the SQUEEZER
and skimmer were set as follows:

Vi = +10.2 V, V2 = -5.54 V, V3 = -2.24 V, andV,k = -10.0 V. The linewidth of the LIF spectrum was 0.85 GHz, which was about 60 %of that for a typical condition of the IGISOL. The deduced energy spread from the LIF
spectrum was 82 eV.
The energy spread is obviously larger than the energy given between the exit
hole and the skimmer. It means that the effect of collisions after the skimmer is not
negligible.

To reduce the collision with helium atoms, a differential pumping systemshould be introduced to obtain better vacuum after the skimmer. By improving thissystem, we expect that the energy spread can be reduced to less than 20 eV, whichcorresponds to a linewidth of about 200 MHz.

4.5

He-gas pressure dependence of the velocity spreadTo investigate the He-gas pressure dependence of the velocity spread, measure-ment was carried out at V,k = -500 V, which is generally used for an ordinary ion guide.
The results of the measurement are shown in fig. 15. The linewidth of the LIF spectra
decreases sharply below 200 mbar with decreasing the Ptc and is nearly constant above
200 mbar.
To obtain a narrower energy spread, operation of the IGISOL with low gas-cellpressure is needed.

However, the efficiency of the IGISOL decreases for low gas-cellpressure [16]. Therefore it is difficult to reduce the helium gas pressure in the gas cell somuch.
The energy spread of the ion beam from the IGISOL is caused by collisions
with helium atoms outside the gas cell. We have, therefore, two ways to reduce the
29

2.0

,.........

Ysk= -500 Y
e = 180 0

1.5

61

N

150

~

l?

~

100

1.0

~

;:J
<J

~

~

~

>

50

0.5

0.0 ----~~~~~~~~~~--~Oo 100 200 300 400 500
Pressure [mbar]
Figure 15: Gas pressure (Ptc ) dependence of the linewidth of an LIF spectrum observed in
the collinear geometry. A deduced instability of the acceleration potential (~U) is givenon the right ordinate.
energy spread. One is the improvement of the vacuum system. The other is to transport
radioactive ions with low potentials, i.e., SQUEEZER.

4.6

Comparison with the mass resolving power
The skimmer-potential and the pressure dependences of the mass resolving power(MRP) were also measured. The mass spectrum was measured at the focal point of theanalyzing magnet using a Faraday-cup, which is placed just behind a l-rnm slit as shownin fig. 8. The values of the U/ ~U are deduced from the results of the measurementof MRP and Doppler broadening using eqs. (13) and (16), and the results are shownin fig. 16. In both cases, the values of the U/ ~U deduced from the MRP and Doppler
broadening have the same tendency, and the absolute value of the U/ ~U deduced fromthe Doppler broadening is about two times larger than that from the MRP.

30

(a) Skimn1er potential dependence of the U/~U

SOO

LIF

400
~

20 mbar
100mbar
20 mbar
100mbar

0

o
MRp·
•

300

<J

"~ 200
100
0
0

200
400
600
800
Skimmer Voltage [-Vl1000(b) He-gas pressure dependence of the U/~U

400---------------------------.
Vsk=-500V
300

o

LIF

•

MRP

~

~ 200

~

•

100

0L-~~--~~----~~~~--~

o

100

200

300

400

500

Pressure [mbar]

Figure 16: Skimmer-potential and pressure dependences of the U/ ~U deduced from themass resolving power and the Doppler broadening: (a) skimmer potential CV,k) dependence; and (b) He-gas pressure (Pte) dependence.

31

To elucidate the reason for the difference, measurement of the Doppler broadeningof the LIF spectrum was carried out using a 1-mm slit at the focal point of the analyzing
magnet. The momentum dispersion at the focal point of the analyzing magnet is 20 mm
per percent. The distance (l [mm]) between separated ion beams with different momenta
at the focal plane of the analyzing magnet is given bydP
l = (20 mm) x p x 100,

(17)where P is a momentum of the ion beam and dP difference of the momentum. (Seefig. 17.) Therefore the Ar 1 + ion beam passing through the 1-mm slit should have anenergy spread of 1000 in terms of U/ ~U.

U
1P
1(20mm)M RP = ~U = "2 dP = 2lx 100 = 1000.

The energy spread of U/ ~U = 1000 corresponds to a linewidth of an LIF spectrum of
0.3 GHz. Figure 18 shows a mass spectrum observed at the focal point of the analyzing
magnet and LIF spectra of the Ar 1+ ion beams, which passed through the 1-mm slit at
the focal point. The parts of the ion beams were selected by changing the magnetic fieldof the mass analyzer. The observed LIF spectra and corresponding parts of the mass
spectrum are shown in fig. 18: (a) the magnetic field was fixed at the peak of the mass
spectrum; and (b,c) the field was fixed at the half maximum. This experiment was carriedout under the usual condition of the IGISOL, i.e., ~k = -500 V and Pte = 100 mbar.
In this experiment, we found a discrepancy between experimentally observed
32

dP
P

MD = 1 ( - • 100)

P, P' ( = P + dP)-1

Figure 17: The definition of the momentum dispersion (MD) of an analyzing magnet.
The value l is a distance of analyzed ion beams of momenta p and p'( = P + dP) at the
focal plane.
linewidth and the linewidth predicted from the mass spectrum: first, the observed Doppler
broadening is much larger than the predicted value (0.3 GHz), which is deduced from thesize of the slit at the focal point using the momentum dispersion of the analyzing magnet;
second, a difference of the peak position of the LIF spectrum associated with the parts (b)and (c) is about two times smaller than that deduced from the MRP (the MRP is 113 andthe expected value is 2.7 GHz), and the observed value is very close to the value of thelinewidth observed without the slit (about 1.4 GHz). However, the prediction was madeusing eq. (16) that is obtained by assuming that the MRP depends on the momentum
spread only. Accordingly, it is understandable that the value of the U/ ~U derived fromthe mass spectrum is smaller than that from the LIF spectrum. Here, we conclude thatthe value of U/ ~U deduced from the MRP is considerably underestimated.

33

D-magnet Current(digital voltmeter reading [mV])
162.0

161.6

161.2

160.8

160.4

Mass Spectrum

3.0

160 .0Ar 1+

FWlIM(MRP=113.4)

2.0
1.0

0.0 L_oI.---.J...:...----...~::.l....l-~:.l...---L.-~---..L.---'--~____,

2500 __------------L-----~------~-----I(a) Peak
FWHM= 1.2 GHz
I
2000

I
I
I
I
I
I

1500
1000

\

500

~

ot

o
o
o

co
o

o
o

\

00
III

~~~~\
\,

)

trI

c::

~

1500

c::
......

(b) Higher
Momentwn
Side

1000

F\VHM= 1.2 GHz

500

oI

I

I

1500
(c) Lower
Momentum
Side

1000

F\VHM=1.5 GHz

500
0
50

100

150
Channel(Laser Frequency [56 MI-f1Jch])

200

Figure 18: The mass spectrum of Ar 1+ ions and LIF spectra which correspond to theparts of the ion beams selected with a I-rum slit at the focal point of the analyzing
magnet: (a) LIF spectrum corresponds to the part a (peak) of the mass spectrum; (b)LIF spectrum corresponds to the part b (higher-momentum side) of mass spectrum; and(c) LIF spectrum corresponds to the part c (lower-momentum side) of the mass spectrum.
34

4.7

Velocity distribution of a collimated beam
In order to get a narrower velocity spread, a pair of hole slits with a hole size of

5 mm in diameter and 80 cm apart was set after the magnetic analyzer. LIF spectra of
Ar 1 + were measured at 90° and 150° with Ptc == 20 mbar, V"k == -500 V and U == 30 kV
at the 0° beam line. The linewidth of the LIF spectrum was 0.68 GHz at 150° incidence
and 1.1 GHz at 90° incidence. From these results the linewidth of the collinear spectrum
for Arl+ is estimated to be 0.23 GHz, which corresponds to 22-eV energy spread. We
successfully obtained ion beams with a narrow energy spread, however, in this case welost about 80 % of the ion beam. The loss seems considerably large to perform on-line
collinear laser spectroscopy. Therefore, generally speaking, this method is not suitable forthe on-line experiment.

4.8

Mass dependence of the MRP
We have measured the MRP using various rare gases. The results are shownin fig. 19. We have found that the MRP changes as a function of mass ~1. It meansthat the energy spread of the ion beam depends on the mass. The MRP observed seems
to be proportional to v'M, i.e., the energy spread ~E becomes smaller for larger mass.
The measurement was done, however, only for three different masses. To discuss massdependence of the energy spread of ion beams from the IGISO L, the effect of the emittanceshould also be considered for each mass. Therefore we shall not discuss about the massdependence of the energy spread here in detail.

35

500
o normal IGISOL

o with SQUEEZER

400
~

300

~

~

200
100

o

o

2

4

6

8

10

12

14

v'K1
Figure 19: The mass dependence of the MRP of the IGISOL. The MRP was measured
for 4He,40 Ar and 136 Xe. The abscissa is a square root of mass M. The open squares were
measured with a normal IGISOL, and the open circles with a SQUEEZER.

36

Table 3: Expected yield of radioactive Hf isotopes from GARIS/IGISOL. The values marked by an asterisk (*) are estimated from experimental data obtained for the
141 Pr(40 Ar, 5n)176 Ir reaction [16].

Reaction

9 Bee 66 Er, xn)175-x H j

Beam current
Cross sec tion
Target thickness
GARIS efficiency
IGISO L window
IGISOL efficiency

1 pnA
1 b (for x == 4 - 9)
3 mg/cm 2
100 %40 % *

Yield of a Hf isotope

4.9

f'.I

1%*
f'.I

5 x 10 3

Yield of radioactive isotopes from the GARISjIGISOL
The expected yield of radioactive Hf isotopes at the GARIS/IGISOL facility ispresented in table 3. The cross sections for these fusion reactions calculated using the
ALICE code is presented in fig. 20 [29]. The effective target thickness is assumed to beabout 3 mg/cm 2 . Because the reaction is of inverse kinematics, the transmission efficiencyof the GARIS is expected to be almost 100 %. At the entrance window of the gas cell of
the IGISOL, about 40 % of the products pass through the window because the beam sizeat the focal point of the GARIS is larger than the size of the window [16]. The extraction
efficiency of the IGISOL is about 1 % [16]. The yield of a Hf isotope is thus estimatedto be 5 x 10 3 particles/s for the fusion reaction 9 Bee 66 Er, xn)175-x H j, x == 4 - 9, at an
166Er beam current of 1 pnA. (Actual beam intensity is expected to be 5 pnA or evenmore.)

37

10000r-------------------------~,..--,

1000

o 172Hf (3n).DEo 171Hf(4n)

'--'

C

.2
.....
u
~
en
en
en
0
:...

U

h. 170Hf (5n)100

• 169Hf (6n)• 168Hf (7n)~ 167Hf (8n)

10

~oo

x 166Hf (9n)

1500

2000

2500

3000

Incident energy [MeV]
Figure 20: The cross sections of the fusion reactions for 9Be(166 Er, xn)175-x H j, which
are calculated using the ALICE code [29]'The observed yield of 169Hf (extracted from the GARIS /IGISOL and accelerated
to 30 keY) was about 100 particles/s for an 166Er beam current of 1 pnA. The helium gas
pressure used in the gas cell was Ptc = 600 mbar.
The observed yield was considerably smaller than the estimated one. There areseveral reasons accounting for this reduction: (1) the He-gas pressure in the gas cell was
too low to stop all of the produced 169Hf ions; and (2) the IGISOL system was not yet
completely optimized. These problems will be solved in due course.

4.10

Feasibility of on-line collinear laser spectroscopy
We have systematically measured the velocity spread of the ion beam from the

IGISOL. The linewidth of the spectrum for Arl+ at the collinear geometry was about 1.4
GHz for the typical IGISOL condition: Ptc = 100 mbar, V,k = -500 V, and U = 30 kV.

It seems still inadequate to do collinear spectroscopy. For heavier element, however, the38

effect of mass M should be taken into consideration. The Doppler broadening ~V D is afunction of the mass and kinetic energy for a given resonance frequency Vo·~VD = vo~j3 =

1 Vo

~E

v'!JE'
v2 c MEM-( 18)

Therefore, the LIF spectrum of the ion beam of a heavier mass with higher kinetic energy
has narrower Doppler broadening. As mentioned in section 4.8, the ~E decreases withincreasing mass. By taking the ~E = canst. = ~E(40 Ar) for M > 40 to estimate safely,
we have a narrower linewidth of the LIF spectrum for a heavy-mass isotope with higher
kinetic energy. For example, a linewidth of about 500 MHz is expected for M= 160

and E = 50 keY. Thus we conclude that in the region from Lu to Ir, collinear laser
spectroscopy with the RIKEN GARIS/IGISOL is quite feasible without a SQUEEZER.
As shown in section 4.4, the new focusing device SQUEEZER was successfully
introduced. The energy spread observed was about 80 eV, and we can reduce it to lessthan 20 e V by introducing a differential pumping system. Therefore, for heavy isotopes
with M > 100, we expect the resonance linewidth of about 100 MHz at U = 50 kY. This
linewidth is narrow enough to carry out on-line collinear laser spectroscopy.
The estimated yield of a Hf isotope is 5 x 10 3 particles/s for a fusion reactionof 9 Be(166 Er, xn)17S-x H f at an 166Er beam current of 1 pnA, as shown in section 4.9.
Since the actual 166Er incident beam intensity from the RIKEN ring cyclotron facility isa few pnA, we can expect a few times more intense radioactive isotope beam. Generally
speaking, however, such a low yield requires detection techniques of higher sensitivity. A
number of rich techniques have been developed [7].

39Acceleration electrodes

Electrostatic
quadrupole doublet

IIIII
CJ CJ
IIIII

ion beams c:J c::::JLenses;FiltersPM

160-m
opticalfiber

Figure 21: The arrangement for coincidence collinear laser spectroscopy. The arrival ofatoms is detected with a Ceratron that detects the secondary electrons emitted when theparticle strikes on an aluminum-coated mirror.
The sensitivity of the collinear laser spectroscopy is limited mainly by backgroundfrom stray laser light, radioactivity, noise of photomultiplier, and so on [7]. To suppress the
background, a photon-particle coincidence method was developed by a group of Daresbury
Laboratory [30]' The sensitivity of the collinear laser spectroscopy with this system wasincreased by several order of magnitude [30]. In fact, they successfully made collinear
laser spectroscopy with a 87Sr beam of about 100 particles/s [31]. Figure 21 shows our
experimental arrangement for the photon-particle coincidence method. This system isready to be tested at RIKEN.

40

5Conclusion
We have constructed an 1SOL system GAR1S/1G1SOL at R1KEN. To investigate

the properties of the ion beam from the 1G1SOL, direct measurement of the velocity
spread of ion beam was systematically made using a method of laser spectroscopy.
The velocity spread was found to be about two times narrower than that estimated from the MRP. It is understood that the energy spread deduced from the MRP
is overestimated, which is caused by the assumption that the mass spectrum depends onthe momentum spread only, while the emittance of the ion beam also contributes to the
MRP.
The line width of the LIF spectrum observed for Arl+ was about 1.4 GHz atpresent for a usual IGISOL condition: V"k = -500 V, Ptc = 100 mbar, and U = 30 kV.
The energy spread corresponding to the linewidth is 135 e V. We have shown that forheavy isotopes with a high kinetic energy, the linewidth will become narrower and on-line
collinear laser spectroscopy is possible. It is also shown that the SQUEEZER works to
reduce the velocity spread of the ion beam significantly. The effectiveness of introducing
a differential pumping system is pointed out.
From the experimental data and theoretical consideration, the experimental
linewidth of an L1F spectrum is about 100 MHz for a radioactive isotope with a mass

M > 100 at U = 50 kV. This linewidth is narrow enough to perform high-resolution laser
spectroscopy of atomic nuclei.
The inverse kinematics of fusion reactions, 9Be(166 Er, xn)175-x H j, was exploited
to produce radioactive Hf isotopes. The radioactive isotope 169Hf was successfully extracted from the GARIS/IG1SOL and accelerated up to 30 ke V. The yield of 169Hf was
41

about 100 particles/s for a beam current of 1 pnA. By optimization of GARIS/IGISOL,
it is expected that the yield will be several ten times larger than that presently observed.
This yield of the radioactive isotopes is much enough to make collinear laser spectroscopy
when we apply the coincidence method [30] .
We thus conclude that the collinear laser spectroscopy of radioactive refractoryelements with the GARIS/IGISOL is promising.

42

Appendix A
Collinear laser spectroscopy setup for on-line experiment
We have constructed a setup for collinear fast atomic-beam laser spectroscopy
and placed it at the 0° beam line of the IGISOL. To establish this system, we made acollinear laser spectroscopy with stable Hf isotope. A schematic diagram of this system
is shown in fig. 22. Instead of the ion-guide gas cell, a Hf metallic plate was placed at theposition of the exit hole of the gas cell, and Hf ions were produced by the irradiation of
the high power frequency-doubled output of a pulsed Nd:YAG laser (LUMONICS mini-Q
Nd:YAG laser). The Nd:YAG laser used was at a wavelength of 523 nm and with an output
power of about 5 m] at a repetition rate of 10 Hz. The ions produced were extracted
through a collimator, accelerated up to 30 keY and separated with an analyzing magnet.
The mass-separated ion beam was transported to the collinear laser spectroscopy system,
which consists of a set of acceleration electrodes for Doppler tuning, a charge exchange
cell for neutralization of the ion beam and an LIF detection system. The neutralizedatomic beam and the laser beam were superimposed collinearly.
The 618.51-nm transition from the 5cf26s 2 3 F2 ground state to the 5d2 6s6p 3 D2state in Hf atoms was observed. According to A. Werner and D. Zimmermann [32] , theexcited Hf atom decays predominantly to the Scf26s 2 3 F3 state, thereby emitting a photonof 724.09-nm wavelength. The energy levels associated with the measurement is shownin fig. 23. A band-pass filter (Kenko BP-72) and two sharp-cut filters (Kenko R-64 andR-66) were set in front of the cooled photomultiplier (Hamamatsu R943-02) in order tomeasure the fluorescence light selectively and to reduce the background . The signal fromthe photomultiplier was recorded as a function of time using a microcomputer (NEC43

Hf Sample
Collim ator
Nd-YAGr'~
I ___ Einzel Lens

I-- ==/
I

Hfion
beams

I

Accelerator
Acceleration electrodes

DD
DD

Charge exchange cell

I

F.~~

o

0

0

0

11111
11111
Lenses

Focal point

160-m
opticalfiber

Figure 22: A schematic diagram for collinear laser spectroscopy.

44---,--3 F2
5d 26s 2

Figure 23: Energy levels of Hf associated with the optical measurement.
PC9801 VX). The LIF spectra of Hf isotopes were measured by scanning the frequency ofthe dye laser.
Figure 24 shows a mass spectrum of singly-charged Hf-ion beams produced bymeans of laser ablation. The width of the slit at the focal point of the analyzing magnet
was 1 mID. The mass resolving power (MRP) for the peak of I8°Hf is 250. Figure 25 showsan LIF spectrum of I80H£. The full width at half maximum (FWHM) of this spectrum
was about 1.5 GHz.
The experimental value of 6.U deduced from the MRP of I8°Hf is about 120 e V.
The value of 6.U deduced from the LIF is about two times larger than that from the
MRP. This result is just opposite compared to previous observations (see section 4.6):
In general, the 6.U deduced from the MRP should be larger than that from the LIF
spectrum, since the MRP depends not only on the momentum distribution of ion beams
but also on the emittance. Probably the reason is to be found in the effective sodium
vapor pressure (or the product of pressure x length) in the charge exchange cell. The
effective vapor pressure should be optimized to avoid unwanted collisions which cause
45

0.2
~

c

177Hf

.....
C
a>
L...
L...

:J

u

0.1

0.0

340

342

344

346

348

350

Magnetic field of IGISOL-D(Digital voltmeter reading [mY))

Figure 24: Mass spectrum of Hf isotopes. This spectrum was measured at the focal pointof the analyzing magnet. The width of the silt at the focal point was 1 mm. The beam
current was measured with the Faraday cup behind the slit. The mass resolving power(MRP) for 18°Rf was about 250.
broadening of the resonance linewid tho
In conclusion, it should be emphasized that the apparatus successfully worked
to measure the 18°Rf LIF spectrum, assuring on-line fast atomic-beam laser spectroscopy
with the RIKEN GARIS/IGISOL.

46

8000~----------------------------~

6000
f\VHM ~ 1.5 GHz
C/)~

5 4000ou

2000o ~----~----~--~~~~~~~~~~
o

2

4

6

10
Freq uency difference [GHz]812

Figure 25: Laser-induced fluorescence (LIF) spectrum of isotope-separated Hf atoms. The
magnetic field of the separator was set to maximum transmission of 180Hf. The width of
the slit at the focal point of the analyzing magnet was 1 rnm. The FWHM of the main
peak of 18°Hf is about 1.5 GHz. The small peaks observed at the lower frequency side of
the main peak of 18°Hf are due to contaminations by other Hf isotopes in the ion beam
after mass separation.

47

Appendix B
High-resolution off-line laser spectroscopy setup for refractoryelements
Before making on-line laser spectroscopy, the atomic structure should be wellstudied. An apparatus for an off-line laser spectroscopy will, therefore , play an importantrole in supporting the study of short-lived unstable nuclei. We have thus constructed anapparatus for off-line laser spectroscopy, which is also useful to study long-lived unstable
nuclei as well as stable.
For an atomic beam source, an oven is commonly used, while the oven is impotent
to make atomic beams of refractory elements. For production of the atomic beams ofrefractory elements, a sputtering atomic-beam source equipped with a hand-made highintensity ion gun was constructed for the off-line apparatus. As shown in fig. 26, theion gun consists of a Duoplasmatron and an Einzel lens. The ion beam from the ion gun
strikes a sample target and kicks the sample atoms out of the target. This phenomenon
is called 'sputtering'. The sputtering atomic-beam source is powerful to produce theatomic beams of refractory elements as well as others, because the sputtering process is
independent of chemical and physical properties, and the most of the sputtered particles
are neutral (more than 90 %) [33]. The conditions of the sputtering ion beam from theion gun are easily controlled: the beam intensity, the spot size of the beam on target,
incident energy, species of the sputtering ion, and so on.
At present, we use Ar-ion beam for sputtering. The maximum current of the Ar 1 +ion beam from the ion gun is a few rnA, and the maximum kinetic energy is 20 ke V. The
atomic beam generated by sputtering and the laser beam cross perpendicularly in front of48

\Duoplasmatron

Einzellens\

Atomicbeam

6 VI 50 A(ma,;)

Sample larget
1 Q
1 kQ

200 VISA
(max)

+

20 kV

5 kV

(max)

(max)

Figure 26: A schematic diagram of the sputtering atomic beam source equipped with
Duoplasmatron and Einzellens. The incident angle of the ion beam to the target is about
60° .
the spherical mirror which is 80 rnm in diameter and 60 mm in radius of curvature. The
fluorescence lights from the atoms are collected with the spherical mirror. The solid angleof photon collection is 1.141f sr. To reduce the background photons from the laser beamand the sputtering atomic-beam source, baffles are used. Figure 27 shows a schematic
diagram of the off-line laser spectroscopy setup.
We successfully observed an LIF spectrum of refractory element natHf with thisapparatus. Figure 28 shows the Hf LIF spectrum. The observed transition is the sameas the measurement mentioned in appendix A. (The 618.51-nm transition was observed.)
The set of the filters and the photomultiplier used were also the same. The observed
linewidth of the Hf LIF spectrum is about 20 MHz. This linewidth is good enough tostudy hyperfine structures.

49

Sphericalmmor
Laser beam

Liq. N2 trap

¢TMP
BaffleBaffleLiq. N2 trap

V
DP

Figure 27: A schematic diagram of the off-line laser spectroscopy setup.

50

50000

180Hf
40000

trJ

~

178Hf

30000

=:
:::

0

U

20000

176Hf
10000
0
100

200

300

400

500

600

700

800

Channel Number

Figure 28: An LIF spectrum of stable Hf isotopes. The wavelength observed was 618.51
nm from the 5cf6s 2 3 F2 ground state to the 5cf6s6p 3 D2 state. The linewidth of thespectrum is about 20 MHz.

51

Acknowledgment
The author is very much obliged to many people during the preparation of thisthesis. In particular, he is grateful to Profs. Y. Yoshizawa, 1. Endo and Dr. T. Horiguchi
for their encouragements and enlightening discussions at Hiroshima university. He also
wishes to acknowledge the continuous encouragement of Prof. H. Kamitsubo at RIKEN.
He wishes to thank Prof. T.T. Inamura, who began the project of laser spectroscopy at
RIKEN, for his continuous encouragement, patience, and timely suggestion, and Dr. K.
Morita, who constructed the GARIS/IGISOL, for his advice and help. He greatly appreciates the contribution of Dr. Y. Matumoto at the very beginning of this study, who
is now at Institute for Molecular Science. He expresses deep gratitude to his coworkers:
Prof. M. Takami, Dr. M. Wakasugi and Mr. A. Yoshida from RIKEN; Prof. H. Katsuragawa and Mr. M. Azuma from Toho university; Prof. T. Ishizuka, Dr. 1. Nakamura,and Mr. M. N akaoka from Saitama university; Prof. K. Valli, and Mr. A. Iivonen from
Jyviiskyla university; Prof. S. Matsuki from Kyoto University; Dr. T. Murayama from
Tokyo university of mercantile marine; Dr. K. Shimomura from Osaka university; Dr. T.
Shinozuka from Tohoku university; Dr. I. Sugai from the university of Tokyo; Prof. Y.
Tagishi from the university of Tsukuba. He also wishes to thank Messrs. M. Suzuki, H.
Kosuga, and M. Takayanagi from Toho university for the assistance in the experiments.
He greatly acknowledges the hospitality extended to him during his stay at RIKEN.

52

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561) Velocity distribution of ion beams from the RIKEN IGISOL(IIW IGISOL iJ~ G o).{ ;t ~ ~ -.4 O)~JJt51;{fi) .
M. Koizuml, Y. Yoshida, K. Morita, M. Takami, T.T. Inamura, M. Azuma, H.
Katsuragawa, M. Nakaoka, r. Nakamura, T. lshizuka, T . murayamc, A. livonen, K.
Valli, K. Shimomura, S. Matsuki, and r. Sugai, to be published in Nucl. lnstr. andMeth. (Dec. 1991).
2) Collinear fast atomic-beam laser spectroscopy at IUKEN GARISjIGISOL(EIliJf GAIUSjIGISOL K.J:J It ~ ~~i\%~DJ(.::r-~ v -~-5t:}'t)
M. Koizuml, M. Azuma, T. Horiguchi, T.T. Inamura, T. Ishizuka, A. livonen, H:
Katsuragawa, S. Matsuki, K. Morita, T. Murayama, r. Nakamura, M. Nakaoka, K.
Shimomura, T. Shinozuka, r. Sugai, Y. Tagishi, M. Takami, K. Valli, Iv1. Wakasugi,
and A. Yoshida, presented at the Int. symposium on lasers in nuclear physics (Wako,
1991), and to be published in Hyp. Int.